The present invention relates to nickel-base materials suitable for high temperature applications. More particularly, this invention relates to a nickel-base intermetallic matrix composite material characterized by the presence of the sigma phase in a matrix of the gamma phase, or vice versa, and alloyed to yield a composite material from which high temperature castings and structural coatings can be formed.
High temperature cobalt-base and nickel-base superalloys are used in the manufacture of high-temperature gas turbine engine components, including nozzles, combustors, and turbine shrouds, vanes and blades. Gamma-prime (.gamma.') precipitation-strengthened nickel-base superalloys have found particularly wide use in gas turbine engines because they exhibit a desirable balance of creep, tensile and fatigue crack growth properties at elevated temperatures. These superalloys are generally characterized by gamma-prime intermetallic precipitates, principally Ni.sub.3 (Al,Ti,Nb,Ta), formed in-situ in the gamma (.gamma.) matrix (principally nickel, chromium, tungsten, rhenium, molybdenum and cobalt), and derive desirable properties from the presence of the gamma-prime precipitates and other alloying constituents at their grain boundaries.
While intermetallic compounds such as gamma-prime have the above-noted benefits, other intermetallic compounds can be detrimental to the mechanical properties of a nickel-base superalloy. An example is topologically close-packed (TCP) phases, including the sigma (.sigma.) phase, characterized by plate-like or needle-like precipitates that have a detrimental effect on toughness. Therefore, TCP phases have traditionally been avoided in nickel-base superalloys, particularly for components that will be subjected to high stresses at elevated temperatures. However, limited investigations of sigma-phase nickel-base superalloys have been performed, as reported by E. Grundy in Solidification and Properties of Ni--Cr--Mo and Ni--Cr--W Monovariant Eutectics, Conference on In Situ Composites-III, p. 431 (1979), and as disclosed in British Patent No. 1,496,930 to Grundy.
The Grundy article and patent report the study of planar front, directionally-solidified (DS) gamma-sigma eutectics of various Ni--Cr--Mo ternary alloys. To obtain a planar solidification front resulting in a lamellae alignment of the sigma phase precipitates, solidification was maintained at a rate of about three centimeters per hour or less. Grundy discloses that the evaluated alloys generally exhibited good oxidation resistance, creep rupture resistance and tensile strength at high temperatures, particular those alloys containing about 15 to 30 weight percent molybdenum. A preferred alloy was said to contain, in weight percent, 34 chromium and 20 molybdenum with the balance nickel, and with a sigma volume fraction of about 30 percent. Potential additional constituents were said to be aluminum, titanium, niobium and tantalum, so long as their combined presence did not exceed 6 weight percent, preferably not more than 3 weight percent, of the alloy. Since the publication of the Grundy article and patent, little additional investigations have been pursued to find practical applications for gamma-sigma intermetallic nickel-base alloys.
Because the efficiency of a gas turbine engine is dependent on its operating temperatures, there is a demand for components, and particularly turbine blades, that are capable of withstanding higher temperatures. As the material requirements for gas turbine engine components have increased, various processing methods and alloying constituents have been suggested to enhance the mechanical properties of gamma-prime precipitation-strengthened nickel-base superalloys. In general, advancements in such technologies have been such that the maximum local metal temperatures of components formed from these superalloys are approaching the alloy melting temperatures. Accordingly, in terms of high temperature capability, it is likely that only modest improvements will be possible in the future for gamma-prime precipitation-strengthened nickel-base superalloys.